The development of novel or improved processes for combustion of high hydrogen content fuels has become increasingly important in view of the development of various integrated power generation and fuel synthesis processes, especially where such processes produce fuels with significant hydrogen content. Commercially available gas turbines have typically been developed for the combustion of natural gas, i.e., a methane-rich fuel with high calorific values in the range of from about 800 to about 1200 BTU/scf (British Thermal Units per standard cubic foot, wherein standard conditions are 14.73 pounds per square inch absolute and 60° F.). While such gas turbines have been adapted to burn certain syngas fuels, and more specifically fuels with low calorific value often in the range of from about 100 to about 300 BTU/scf, gas turbine combustor design features have not generally been optimized for hydrogen content or low grade gaseous fuel applications.
Conventional gas turbine engines encounter two basic difficulties when transitioning from natural gas to syngas. First, for the same fuel heat input, the mass flow of a syngas fuel is often four to five times greater than that for natural gas, due to the lower heating value of the syngas fuel. Second, although premixed natural gas and air combustion systems have become common place for controlling NOx emissions, such systems have not been successfully implemented for syngas applications, due to the high hydrogen content of the syngas, and the accompanying potential for flashback of the flame into the fuel injection system. Consequently, diffusion flame or “non-premixed” combustors which have been used in the combustion of syngas have been configured to control the NOx emissions by diluting the syngas with nitrogen, steam or carbon dioxide. In such designs, the diluent reduces the flame temperature and consequently reduces the formation of NOx.
In the combustion of natural gas, dry (i.e., no addition of steam or water) low NOx (DLN, or “Dry Low NOx”) combustors can achieve less than 10 ppmvd (10 parts per million by volume, dry, at 15% Oxygen) NOx emissions with a natural gas fuel. Such DLN combustors rely on the premix principle, which reduces the combustion flame temperature, and consequently the NOx emissions. DLN combustors are able to achieve much lower NOx emissions than diluted non-premixed combustors because of higher premixing time prior to the combustion zone.
In high hydrogen content fuel, such as is found in some syngas mixtures (up to 60% hydrogen by volume or more), or in pure hydrogen fuel sources, the flame speeds may be up to as much as six times faster than the flame speed that is typical in combustion of natural gas. Consequently, such high flame speed mixtures, whether from syngas based fuels or from other hydrogen source fuels, makes the use of a DLN combustion system impossible, because in such a system the flame would flash back into the premix zone, and destroy the fuel injection hardware.
On the other hand, the diluted non-premixed combustors have a chemical kinetic limit when too much diluent is added for reduction of NOx emissions. The increase in diluent causes flame instability in the combustion zone, and eventually, combustor flame-out. Consequently, in the best case, a practical NOx reduction limit for prior art syngas combustors is presently between about 10 and about 20 ppmvd NOx.
In summary, there remains an as yet unmet need for a combustor for a gas turbine engine that may be utilized for the combustion of high hydrogen content fuels. In order to meet such needs and achieve such goals, it is necessary to address the basic technical challenges by developing new system designs. As described herein, advantageous gas turbine system designs may include the use of a lean premix with high hydrogen content fuels in combination with the use of trapped vortex combustors.
The foregoing figures, being merely exemplary, contain various elements that may be present or omitted from actual embodiments which may be implemented, depending upon the circumstances. An attempt has been made to draw the figures in a way that illustrates at least those elements that are significant for an understanding of the various embodiments and aspects of the invention. However, various other elements of a novel trapped vortex combustor, and methods for employing the same in the combustion of high flame speed fuels such as hydrogen rich syngas, may be utilized in order to provide a versatile gas turbine engine with novel trapped vortex combustor for combustion of a fuel-air premix while minimizing emissions of carbon monoxide and oxides of nitrogen.